In recently years, plastic insulating materials are expected as film materials for film capacitors for communication, electronic devices, electric power, medium and low voltage phase advancement and inverter, piezoelectric devices, pyroelectric devices and dielectric materials for carrying transferring substance since they have a high insulation resistance, excellent frequency characteristics and excellent flexibility.
A film capacitor is usually comprised of a film structure comprising a dielectric polymer film subjected to aluminum or zinc deposition on its surface, or a film structure comprising multi-layers of aluminum foils and dielectric polymer films, and recently there are used a lot of film capacitors comprising a dielectric polymer film and an electrode formed thereon by metal deposition.
Hydrocarbon polymers such as polypropylene, polyester and polyphenylene sulfide are studied as a dielectric polymer for a film of film capacitor. However, a dielectric constant of such a film alone is only about 2.3 to about 3.
It is known that a capacity of a film capacitor is proportional to a dielectric constant of a film used and reversely proportional to a film thickness.
Therefore, making a film thinner has been studied, but if a film is made too thin, film formation becomes difficult, and lowering of a withstand voltage is recognized. Accordingly, there is a limit in making a film thinner.
For making a dielectric constant of a film itself higher, highly dielectric polyvinylidene fluoride polymer and cyano-ethylated pullulan have been studied as a polymer. However, dielectric constants of any of these films are not more than 20, and in addition, it is difficult to make the films thinner.
In recent years, making a size of a film capacitor smaller and its capacity larger has been advanced, and as a result, it is strongly demanded to make a dielectric constant of a film capacitor higher.
A proposal of incorporating inorganic particles having an especially high dielectric constant with a polymer to make a film was made as one of means for making a dielectric constant of a capacitor film higher.
For mixing inorganic ferroelectric particles to a polymer and making a film, there are known (1) a melt-kneading method and (2) a coating method.
The melt-kneading method (1) is a method of kneading inorganic ferroelectric particles and a polymer at a temperature of not less than a melting temperature of the polymer, making the mixture into a film by a melt-extrusion method or an inflation method, and if necessary, subjecting the film to stretching treatment. In this method, there are known the use of a hydrocarbon polymer such as polyphenylene sulfide, polypropylene or polyester (JP2000-501549A and JP2000-294447A) and the use of a vinylidene fluoride polymer (for example, JP59-43039A, JP60-185303A and JP58-69252A). However, it is difficult to produce a thin film having a high dielectric constant and few voids.
The coating method (2) is a method of producing a film by dissolving a polymer in a solvent, adding and mixing inorganic ferroelectric particles thereto to make a coating composition, and then forming a film by a coating method.
In the coating method (2), the use of thermosetting hydrocarbon polymers or precursors thereof being excellent in heat resistance and mechanical strength such as aromatic polyamide, aromatic polyimide and an epoxy resin (JP2001-106977A, JP1-248404A, JP4-160705A, JP2-206623A and JP2002-356619A), and the use of a vinylidene fluoride polymer (JP54-129397A) are known.
In the case of using a thermosetting hydrocarbon polymer, it is possible to produce a thin film having high mechanical strength, but a dielectric loss of the film is large and the film becomes hard. Therefore, it cannot be said that such a film is suitable as a film for a film capacitor which is required to have excellent winding property (flexibility). Especially, a system obtained by adding and mixing inorganic ferroelectric particles is very hard, and is used for an embedded capacitor, making use of its property of being hard.
In the case of using a polyimide precursor, heat treatment at 200° to 400° C. for 20 minutes is necessary, and productivity is lowered.
In the case of using a vinylidene fluoride polymer, it is difficult to obtain a film in which inorganic ferroelectric particles are homogeneously incorporated in a vinylidene fluoride polymer, and further improvement in making a film thin and increasing a dielectric constant is required.
Further, it is proposed to produce a sheet comprising highly incorporated inorganic ferroelectric particles by press-molding composite particles prepared by coating inorganic ferroelectric particles with a vinylidene fluoride polymer (JP61-224205A). However, the film is as thick as 150 μm, and cannot be said to be suitable as a film for a film capacitor which is required to have excellent winding property (flexibility).